Role of metalloproteases in the CD95 signaling pathways

Abstract

CD95L (also known as FasL or CD178) is a member of the tumor necrosis family (TNF) superfamily. Although this transmembrane ligand has been mainly considered as a potent apoptotic inducer in CD95 (Fas)-expressing cells, more recent studies pointed out its role in the implementation of non-apoptotic signals. Accordingly, this ligand has been associated with the aggravation of inflammation in different auto-immune disorders and in the metastatic occurrence in different cancers. Although it remains to decipher all key factors involved in the ambivalent role of this ligand, accumulating clues suggest that while the membrane bound CD95L triggers apoptosis, its soluble counterpart generated by metalloprotease-driven cleavage is responsible for its non-apoptotic functions. Nonetheless, the metalloproteases (MMPs and ADAMs) involved in the CD95L shedding, the cleavage sites and the different stoichiometries and functions of the soluble CD95L remain to be elucidated. To better understand how soluble CD95L triggers signaling pathways from apoptosis to inflammation or cell migration, we propose herein to summarize the different metalloproteases that have been described to be able to shed CD95L, their cleavage sites and the biological functions associated with the released ligands. Based on these new findings, the development of CD95/CD95L-targeting therapeutics is also discussed.

Keywords: ADAM; CD95L; MMP; cancer; cleavage; inflammation.

Figure 1

CD95L structures and cleavages sites. (A) Representation of CD95L domains. Proline rich domain: PRD; Casein kinase I substrate motif: CSI; TNF homology domain: THD. (B) Representation of the different cleavage sites described within the CD95L stalk region. (C) Alignment of human and mouse CD95L protein sequence using Clustal omega (1.2.4). The transmembrane and the stalk domains are represented.

Figure 2

Domains in human MMPs and ADAMs. (A) Schematic representation of the domains in human MMPs consisting in a signal peptide, a prodomain, a metalloprotease/catalytic domain, a linker domain, a hemopexin domain, fibronectin inserts, a convertase cleavage site, a membrane linker, a glycosylphosphatidylinositol, a transmembrane segment 1, a cytoplasmic tail, a transmembrane segment 2, a cysteine array and immunoglobulin-like domain. (B) Schematic representation of ADAMs organized in modules consisting in a prodomain, a metalloprotease/catalytic domain, a disintegrin domain, a cysteine rich domain, an EGF-like domain, a transmembrane region and a cytoplasmic tail. (C) Crystal structure of a typical Metalloprotease/catalytic domain in cartoon representation (hMMP-12, PDB code: 4GQL), with catalytic zinc ion as magenta ball, His residues chelating the catalytic zinc ion in yellow stick, catalytic glutamic acid residue in blue stick, and structural zinc and calcium ion as grey and green balls, respectively.

Figure 3

CD95/CD95L-mediated signaling pathways. (Left) Binding of m-CD95L to CD95 induces an apoptotic signaling pathway. (Right) m-CD95L processing by proteases (ADAMs, MMPs, plasmin) leads to the release of different s-CD95L in the extracellular environment. Depending on the ratio m-CD95L/s-CD95L, and the shedding sequence, several signaling pathways can be triggered: cell survival, migration (promotes the development of metastases), chemoattraction and pro-inflammatory signal, or cell death. Blocking of CD95L binding to CD95 by APG101 (Asunercept) blocks both apoptotic and non-apoptotic signaling pathways.